KINETICS OF CARRIER-MEDIATED ALKALI CATION-TRANSPORT THROUGH SUPPORTED LIQUID MEMBRANES - EFFECT OF MEMBRANE SOLVENT, COTRANSPORTED ANION, AND SUPPORT

The rate-limiting step in the transport of alkali cations through supported liquid membranes mediated by calix [4] arene carriers can be the diffusion of the carrier cation complex through the membrane and/or the kinetics of cation release from the complex. The effects of membrane solvent, co-transported anion, and support on the diffusion constant D-m, the extraction constant K-ex and the rate constant k have been studied. These were determined from flux measurements as a function of source phase salt activity (D-m and K-ex) and membrane thickness (D-m and k). Additional information about the transport resistances was obtained from variation of the operating temperature and from lag time measurements. The diffusion constants (D-m) for 1/NaClO4 and 2/KClO4 linearly increased with reciprocal solvent viscosity. On increasing the solvent polarity, the extraction constant increased, while the rate of cation release decreased. Both log K-ex and log k correlate linearly with the Kirkwood function, (epsilon(r)-1)/(2 epsilon(r)+1). The co-transported anion (ClO4- vs. SCN-) affects the kinetics of release but not the diffusion constant. The normalized k and D-m values for 2/KClO4 in NPOE/Accurel(R) 1E-PP and NPOE/Celgard(R) 2500 are nearly the same. This means that the transport regime (diffusion or kinetic control) depends only on the tortuosity (tau) and thickness of the support, irrespective of its morphology.